In a lab at Johns Hopkins University School of Medicine in Baltimore, a blob of human tissue designed to mimic the properties of an intestine swells with fluid as scientists try to bring on symptoms associated with a bout of diarrhea. It’s the first step to determine whether the structure, known as an organoid, can be used to test treatments for this ailment, which the World Health Organization says kills about 1.5 million children a year. The work is one of several similar efforts nationwide that are part of a push to make early product testing cheaper, faster and less prone to failure for Pfizer Inc. (PFE), Merck & Co. (MRK) and other drugmakers. The technology may one day help them spot drug reactions within days, or even hours, said Mark Donowitz, the Johns Hopkins research leader.